Recently, due to development of switching devices capable of withstanding relatively high current and voltage of high frequency, most of power supply circuits that rectify a commercial power supply and obtain a desired direct-current voltage have become switching type power supply circuits.
Switching power supply circuits have a transformer and other devices miniaturized by raising switching frequency, and are used as a power supply for various electronic devices as a high-power DC—DC converter.
Generally, when a commercial power supply is rectified, a current flowing in a smoothing circuit forms a distorted waveform, thus causing a problem in that a power factor indicating efficiency of use of the power supply is degraded.
In addition, a measure to suppress harmonics caused by the distorted current waveform is required.
Accordingly, various switching power supply circuits having a configuration for power factor improvement added thereto are proposed. As one of such switching power supply circuits, a switching power supply circuit using a so-called choke input system is known in which a power choke coil is inserted in series with a commercial alternating-current power supply line to thereby increase a conduction angle of alternating input current and thus improve a power factor (see Japanese Patent Laid-Open No. Hei 7-263262 (FIG. 19)).
FIG. 27 shows an example of a switching power supply circuit configured to improve a power factor by a choke input system as described above. In this power supply circuit shown in this figure, a power factor improving configuration of the choke input system is added to a configuration of a complex resonant converter previously proposed by the present applicant.
Incidentally, the power supply circuit shown in this figure meets conditions of [load power Po=150 W or higher and an alternating input voltage VAC=100 V system].
The power supply circuit shown in this figure has a common mode noise filter formed by connecting a common mode choke coil CMC and two across capacitors CL to a commercial alternating-current power supply AC. The common mode noise filter suppresses noise transmitted from a switching converter side to the commercial alternating-current power supply AC, for example.
A rectifying and smoothing circuit comprising a bridge rectifier circuit Di and a smoothing capacitor Ci is provided for a line of the commercial alternating-current power supply AC as shown in the figure. This rectifying and smoothing circuit is supplied with the commercial alternating-current power supply AC and then performs a rectifying and smoothing operation, whereby a rectified and smoothed voltage Ei having a level corresponding to the alternating input voltage VAC multiplied by unity is obtained across the smoothing capacitor Ci. The rectified and smoothed voltage Ei is supplied as direct-current input voltage to a switching converter in a succeeding stage.
As a configuration for power factor improvement, a power choke coil PCH is inserted in series with the line of the commercial alternating-current power supply AC. In this case, the power choke coil PCH is inserted in a negative electrode line of the commercial alternating-current power supply AC.
When the power choke coil PCH is thus inserted in the line of the commercial alternating-current power supply AC, as is well known, an inductance Lpch of the power choke coil PCH acts to suppress harmonics of an alternating input current flowing into rectifier diodes forming the bridge rectifier circuit Di from the commercial alternating-current power supply AC. That is, a conduction angle of the alternating input current IAC is increased to improve the power factor.
The power supply circuit shown in this figure has a complex resonant converter as switching converter that is supplied with the above-mentioned rectified and smoothed voltage Ei for operation. The complex resonant converter in this case refers to a switching converter having a resonant circuit added to a primary side or a secondary side thereof in addition to a resonant circuit provided to convert operation of the switching converter into a resonant type operation, so that the plurality of resonant circuits are operated in a complex manner within one switching converter.
The resonant converter provided as the complex resonant converter in the power supply circuit shown in FIG. 27 is of a current resonant type. The current resonant converter in this case has two switching devices Q1 and Q2 formed by a MOS-FET connected to each other by half-bridge coupling as shown in the figure. Damper diodes DD1 and DD2 are connected in a direction shown in the figure in parallel with the switching devices Q1 and Q2 between a drain and a source of the switching devices Q1 and Q2, respectively.
A partial resonant capacitor Cp is connected in parallel with the switching device Q2 between the drain and the source of the switching device Q2. A capacitance of the partial resonant capacitor Cp and a leakage inductance L1 of a primary winding N1 form a parallel resonant circuit (a partial voltage resonant circuit). Then, a partial voltage resonant operation, in which voltage resonance occurs only when the switching devices Q1 and Q2 are turned off, is obtained.
The power supply circuit is provided with a control IC 2 to switching-drive the switching devices Q1 and Q2. The control IC 2 includes an oscillating circuit for driving the current resonant converter by an external excitation system, a control circuit, a protecting circuit and the like. The control IC 2 is a general-purpose analog IC (Integrated Circuit) having a bipolar transistor therewithin.
The control IC 2 operates on a direct-current voltage input to a power supply input terminal Vcc. The rectified and smoothed voltage Ei is input as a starting voltage to the power supply input terminal Vcc via a starting resistance RS. The control IC 2 is started by the starting voltage input to the power supply input terminal Vcc at a time of a start of power supply.
The control IC 2 has two drive signal output terminals VGH and VGL as terminals for outputting drive signals (gate voltages) to the switching devices.
The drive signal output terminal VGH outputs a drive signal for switching-driving the high side switching device. The drive signal output terminal VGL outputs a drive signal for switching-driving the low side switching device.
In this case, the drive signal output terminal VGH is connected to a gate of the high side switching device Q1. The drive signal output terminal VGL is connected to a gate of the low side switching device Q2.
Thus, the high side drive signal output from the drive signal output terminal VGH is applied to the gate of the switching device Q1. The low side drive signal output from the drive signal output terminal VGL is applied to the gate of the switching device Q2.
Though not shown in the figure, a bootstrap circuit as an external circuit is connected to the control IC 2. This bootstrap circuit shifts level of the high side drive signal output from the drive signal output terminal VGH to a level at which the switching device Q1 can be driven properly.
The control IC 2 generates an oscillating signal of a required frequency by the internal oscillating circuit. Then, the control IC 2 generates the high side drive signal and the low side drive signal using the oscillating signal generated by the oscillating circuit. The high side drive signal and the low side drive signal are generated in such a relation as to have a 180° phase difference from each other. Then, the control IC 2 outputs the high side drive signal from the drive signal output terminal VGH, and outputs the low side drive signal from the drive signal output terminal VGL.
The high side drive signal and the low side drive signal are applied to the switching devices Q1 and Q2, respectively. In a period of an H level of the drive signal, the gate voltage of the switching devices Q1 and Q2 is higher than a gate threshold value, and therefore the switching devices Q1 and Q2 are in an on state. In a period of an L level of the drive signal, the gate voltage of the switching devices Q1 and Q2 is lower than the gate threshold value, and therefore the switching devices Q1 and Q2 are in an off state. Thus the switching devices Q1 and Q2 are switching-driven at a required switching frequency in timing in which the switching devices Q1 and Q2 are turned on/off alternately.
The isolated converter transformer PIT is provided to transmit a switching output of the switching devices Q1 and Q2 from the primary side to the secondary side.
In this case, one end of the primary winding N1 of the isolated converter transformer PIT is connected to a point of connection (a switching output point) between the switching devices Q1 and Q2 via a primary side series resonant capacitor C1. Another end of the primary winding N1 is connected to a primary side ground.
A capacitance of the series resonant capacitor C1 and the leakage inductance (L1) of the primary winding N1 form a primary side series resonant circuit. This primary side series resonant circuit is supplied with the switching output of the switching devices Q1 and Q2 and thereby performs resonant operation. Thereby the primary side series resonant circuit converts the operation of a switching circuit comprising the switching devices Q1 and Q2 into a current resonant type operation.
Thus, the primary side switching converter in the circuit shown in FIG. 27 obtains the current resonance type operation by the primary side series resonant circuit (L1–C1) and the partial voltage resonant operation by the partial voltage resonant circuit (Cp//L1) described above.
That is, the power supply circuit shown in this figure employs the form of the complex resonant converter in which the resonant circuit for making the primary side switching converter a resonant converter is combined with another resonant circuit.
A secondary winding N2 is wound on the secondary side of the isolated converter transformer PIT.
The secondary winding N2 in this case is provided with a center tap, which is connected to a secondary side ground. A full-wave rectifier circuit comprising rectifier diodes D01 and D02 and a smoothing capacitor C0 is connected to the secondary winding N2. Thereby a secondary side direct-current output voltage E0 is obtained as a voltage across the smoothing capacitor C0. The secondary side direct-current output voltage E0 is supplied to a load side not shown in the figure, and also branches off to be input as a detection voltage for a control circuit 1.
The control circuit 1 supplies, as a control output, a current or a voltage varied in level according to level of the secondary side direct-current output voltage E0 input to the control circuit 1 to a control input terminal Vc of the control IC 2. The control IC 2 varies the frequency of the oscillating signal, for example, according to the control output input to the control input terminal Vc, and thereby varies the frequency of the drive signals to be output from the drive signal output terminals VGH and VGL. Thereby the switching frequency of the switching devices Q1 and Q2 is variably controlled. By thus varying the switching frequency, the level of the secondary side direct-current output voltage E01 is controlled to be constant. That is, stabilization by a switching frequency control system is performed.
FIG. 28 shows, by solid lines, characteristics of the power factor PF, power conversion efficiency ηAC→DC, and the level of the rectified and smoothed voltage Ei (direct-current input voltage) in a load variation range of load power Po=150 W to 0 W when the alternating input voltage VAC=100 V in the case of the power supply circuit shown in FIG. 27.
For comparison, characteristics in a case where the power supply circuit shown in FIG. 27 does not have a power factor improving configuration are shown by broken lines. That is, characteristics when the component of the inductance Lpch of the power choke coil PCH is omitted from the line of the commercial alternating-current power supply AC are shown by the broken lines.
FIG. 29 shows characteristics of the power factor PF, the rectified and smoothed voltage Ei, and the power conversion efficiency ηAC→DC in a voltage level variation range of the alternating input voltage VAC=80 V to 120 V when the load power Po=150 W in the case of the power supply circuit shown in FIG. 27.
In obtaining experimental results shown in FIG. 28 and FIG. 29, parts of the power supply circuit shown in FIG. 27 are selected as follows.
The power choke coil PCH Lpch=10 mH
The isolated converter transformer PIT: an EER35 ferrite core, a gap length of 1 mm
The primary winding N1=25 T
The secondary winding N2: 23 T+23 T with a center tap as a dividing position
The primary side series resonant capacitor C1=0.082 μF
The partial resonant capacitor Cp=680 pF
Parts of the power supply circuit exhibiting the characteristics indicated by the broken lines in FIG. 28, the power supply circuit being formed by omitting the power choke coil PCH (inductance Lpch) from the circuit of FIG. 27, are changed as follows.
The isolated converter transformer PIT: an EER35 ferrite core, a gap length of 1 mm
The primary winding N1=31 T
The secondary winding N2: 23 T+23 T with a center tap as a dividing position
The primary side series resonant capacitor C1=0.068 μF
The partial resonant capacitor Cp=680 pF
As shown in FIG. 28, the power conversion efficiencies ηAC→DC indicated by a solid line and a broken line both have a tendency to increase as the load power is increased. The characteristic of power conversion efficiency of the circuit shown in FIG. 27 in which the inductance Lpch is inserted, which characteristic is indicated by the solid line, reaches a maximum of ηAC→DC=87.5% when the load power Po=150 W.
The rectified and smoothed voltages Ei indicated by a solid line and a broken line are both decreased gently as the load becomes heavier. The characteristic of the rectified and smoothed voltage Ei when the inductance Lpch is inserted, which characteristic is indicated by the solid line, shows a change of Ei=134 V→115 V with respect to variation of the load power Po=0 W→150 W.
The power factor PF is increased as the load power is increased, and becomes substantially flat when the load power Po=75 W or higher. When the load power Po=150 W, the power factor PF=0.75.
Also, according to FIG. 29, the power factor PF is constant at about 0.75 with respect to variation of the alternating input voltage VAC. The power conversion efficiency ηAC→DC has a tendency of increasing gently as the alternating input voltage VAC rises. The rectified and smoothed voltage Ei rises in substantial proportion to the alternating input voltage VAC.
FIG. 30 shows another example of a complex resonant converter configured to improve a power factor by a choke input system. The power supply circuit shown in this figure can meet conditions of [load power Po=250 W or higher and an alternating input voltage VAC=100 V system]. Incidentally, in this figure, the same parts as in FIG. 27 are identified by the same reference numerals, and description thereof will be omitted.
The power supply circuit shown in this figure meets the condition of a heavier load than the power supply circuit of FIG. 27. Hence, a voltage doubler rectifier circuit is provided as a rectifying and smoothing circuit for generating a rectified and smoothed voltage Ei. The voltage doubler rectifier circuit in this case is formed by connecting two rectifier diodes Dia and Dib and two smoothing capacitors Ci1 and Ci2 connected in series with each other to an commercial alternating-current power supply AC, as shown in the figure.
The voltage doubler rectifier circuit is supplied with the alternating input voltage VAC and performs a rectifying and smoothing operation, whereby the rectified and smoothed voltage Ei corresponding to twice a level of the alternating input voltage VAC is generated across the series connection circuit of the smoothing capacitors Ci1→Ci2.
A primary side switching converter in a succeeding stage is supplied with the thus generated rectified and smoothed voltage Ei as direct-current input voltage, and performs switching operation.
FIG. 31 shows, by solid lines, characteristics of the power factor PF, power conversion efficiency ηAC→DC, and the level of the rectified and smoothed voltage Ei (direct-current input voltage) in a load variation range of load power Po=300 W to 0 W in the case of the power supply circuit shown in FIG. 30.
For comparison, also in this figure, characteristics in a case where the power supply circuit shown in FIG. 30 does not have a power factor improving configuration (in a case without a power choke coil PCH (inductance Lpch)) are shown by broken lines.
FIG. 32 shows characteristics of the power factor PF, the rectified and smoothed voltage Ei, and the power conversion efficiency ηAC→DC in a variation range of the alternating input voltage VAC=80 V to 120 V when the load power Po=300 W.
In obtaining experimental results shown in FIG. 31 and FIG. 32, parts of the power supply circuit shown in FIG. 30 are selected as follows.
The power choke coil PCH Lpch=5 mH
An isolated converter transformer PIT: an EER35 ferrite core, a gap length of 1 mm
A primary winding N1=28 T
A secondary winding N2: 25 T+25 T with a center tap as a dividing position
A primary side series resonant capacitor C1=0.039 μF
A partial resonant capacitor Cp=680 pF
The power supply circuit formed by omitting the power choke coil PCH (inductance Lpch) from the circuit of FIG. 30 is changed as follows. The power supply circuit thus formed exhibits the characteristics indicated by the broken lines in FIG. 31.
The isolated converter transformer PIT: an EER42 ferrite core, a gap length of 1 mm
The primary winding N1=32 T
The secondary winding N2=25 T
The primary side series resonant capacitor C1=0.033 μF
The partial resonant capacitor Cp=680 pF
As shown in FIG. 31, the power conversion efficiencies ηAC→DC indicated by a solid line and a broken line are both substantially constant in a range of the load power Po=100 W and higher. The characteristic of power conversion efficiency of the circuit shown in FIG. 30 in which the inductance Lpch is inserted, which characteristic is indicated by the solid line, indicates ηAC→DC=91.1% when the load power Po=300 W.
The rectified and smoothed voltages Ei indicated by a solid line and a broken line are both decreased gently as the load becomes heavier. The characteristic of the rectified and smoothed voltage Ei when the inductance Lpch is inserted, which characteristic is indicated by the solid line, shows a change of Ei=264 V→244 V with respect to variation of the load power Po=0 W→300 W.
The power factor PF has a tendency to increase as the load power is increased. When the load power Po=300 W, the power factor PF=0.75.
Also, according to FIG. 32, while the power factor PF is decreased gently as the alternating input voltage VAC becomes higher, it can be said that with a gradient of this degree, the power factor PF is constant at about 0.75 with respect to variation of the alternating input voltage VAC. The power conversion efficiency ηAC→DC has a tendency of increasing gently as the alternating input voltage VAC rises. The rectified and smoothed voltage Ei rises in substantial proportion to the alternating input voltage VAC.
As described thus far, the power supply circuits shown in FIG. 27 and FIG. 30 improve the power factor by the choke input system. Thereby a value of the power factor PF at a sufficient level to meet a power supply harmonic distortion regulation value for color television receivers, for example, is obtained.
However, the power supply circuits having the configurations shown in FIG. 27 and FIG. 30 have the following problems.
The power choke coil PCH provided for power factor improvement in the power supply circuits of FIG. 27 and FIG. 30 is formed by for example a silicon steel sheet core and a copper wire winding. Hence, there occur a core loss at the core and a copper loss caused by resistance of the copper wire, and therefore a power loss at the part of the power choke coil PCH is correspondingly increased.
Also, an inductance and a resistive component of the choke coil cause a voltage drop in the alternating input voltage VAC. Thereby the direct-current input voltage (rectified and smoothed voltage Ei) obtained by rectifying the alternating input voltage VAC is also lowered.
Thus, power conversion efficiency of the complex resonant converter operating on the direct-current input voltage input thereto is decreased, and also alternating-current input power is increased.
In the case of the power supply circuit shown in FIG. 27, for example, while the power factor PF is improved from 0.55 to 0.75 by inserting the power choke coil PCH, total power conversion efficiency ηAC→DC is decreased by 3.1 percentage points from 90.6% to 87.5%. The alternating-current input power Pin is increased by 5.9 W from 165.5 W to 171.4 W. Incidentally, the rectified and smoothed voltage Ei is lowered by 19 V from 134 V to 115 V.
In the case of the power supply circuit shown in FIG. 30, the power factor PF is improved from 0.60 to 0.75 by inserting the power choke coil PCH. However, the power conversion efficiency ηAC→DC is decreased by 1.7 percentage points from 92.8% to 91.1%. The alternating-current input power Pin is increased by 6.0 W from 320 W to 326.0 W. The rectified and smoothed voltage Ei is lowered by 20 V from 264 V to 244 V.
In addition, the power choke coil PCH has a large size and a great weight among parts constituting the power supply circuit. Therefore the power choke coil PCH occupies a large area on a board, and increases the weight of the circuit board.
When a leakage flux is to be reduced as much as possible in the power choke coil PCH, the core is formed into a ladder-like shape (an E-E-shape or an E-I-shape). For example, the weight and the board occupying area of the power choke coil PCH with such a core of the ladder-like shape are 153 g and 11 cm2, respectively, in the power supply circuit shown in FIG. 27, and 240 g and 19 cm2, respectively, in the power supply circuit shown in FIG. 30.
Further, as described above, the power choke coil PCH causes a relatively large amount of leakage flux. Depending on conditions such as arrangement of parts, the amount of leakage flux and the like, the leakage flux of the power choke coil PCH may affect a load side. In such a case, a part such as a magnetic shield or the like is added as a measure to suppress the leakage flux radiated from the power choke coil PCH, thus increasing the size and weight of the board.
Thus, the power supply circuits configured to improve the power factor by the choke input system have problems in that a decrease in power conversion efficiency, an increase in size and weight of the power supply circuits, and an increase in cost that result from the insertion of the power choke coil are inevitable.